Magnetic encoders help break 7-second motorcycle quarter-mile record

By Control Engineering Staff March 29, 2007

Hoffman Estates, IL —Take a 360 kg motorcycle with absurd power output, a fearless Finn with a reputation for breaking speed records, and things happen fast. Within a split second, the engine screams to 7,500 rpm. Before the spectator has a chance to comprehend what has transpired, the race is over and bike and rider are a quarter mille down the track, braking frantically. In motor racing terms, it doesn’t get much simpler—traverse the straight line from A to B as quickly as possible.

Built by Finland-based Salakazi Racing and generating almost 1,500 hp from just 1,995 cc, the KTM nitro methane dragster does the quarter mile in 6.7 seconds at a terminal velocity of 196 mph (316 km/h). Helping tame the KTM’s (barely) controlled explosion are a brace of “bomb proof” Renishaw RM22 compact, high-speed rotary magnetic encoders.

Jaska Salakari—the first Finn to break the seven-second barrier for the quarter mile piloting the fastest Super Twin Top Fuel bike in the European Union—owns Salakazi Racing. Since 2000, his speed-obsessed six-man team, a combination of full- and part-timers, has been running a KTM Super twin- powered dragster of its own design and construction. The 1,995 cc twin-cylinder Nitroduke engine is fueled with 95% nitro methane, gulping 1.1 liters per second of the volatile mix through a Salakazi-designed double fuel pump. It’s about as close to a flying bomb as any earthbound vehicle could get.

In drag racing, reaction times are measured in milliseconds, and races are never won with a wildly spinning rear wheel. All the horsepower in the world is useless unless effectively translated into rapid forward motion. For the pilot, there’s little or no time to think, let alone engage and slip a normal clutch. It’s impossible for the rider to accurately determine the rate of engagement needed to provide optimal traction.

Salakazi Racing has equipped its KTM dragster with an automatic Prowork three-disc, four-stage clutch fitted with a Prowork digital controller. When the rider snaps open the throttle, the controller engages the clutch according to how it has been pre-programmed. That’s where the Renishaw RM22 encoders with their ability to monitor up to 30,000 rpm come into play. One of the tiny but critical devices monitors the position of the crankshaft in the engine, while the other measures the clutch speed.

By comparing these values, clutch slippage, traction, and road conditions can be determined with high precision. These data are highly coveted by the team and allow the technicians to properly adjust the first-stage counterweights in the clutch before the race.

Of course, track and bike conditions vary for every meeting, but the vital data collected and compiled by the Renishaw encoder after every run makes it possible to program the controller for as close to optimum clutch engagement as possible. This provides maximum speed and acceleration with minimal wheel spin during the first few fractions of a second.

Petri Mäkinen, Salakazi Racing’s self-confessed “technology guru” holds the Renishaw encoder mounted on an access plate that bolts into the clutch housing. (Photo courtesy of Renishaw).

Petri Mäkinen—Salakazi Racing’s self-confessed “technology guru”—is quick to point out that the Renishaw-enabled solution is not the same as a traction control system. The clutch is tuned by set values programmed into the controller before the race against the clock even begins. “The Renishaw encoder isn’t interactively controlling the clutch while underway,” he explains, “but the data it provides us with beforehand is critically important. We couldn’t obtain it—and couldn’t preset the clutch—any other way. We need to read the clutch speed even before the rear tire completes the first full rotation, to make decisions for engaging the clutch, and the acceleration doesn’t happen smoothly. This problem has now been solved and proper filtration has been done to program code.”

Mäkinen uses the data to put the maximum engine power on the road and propel Salakari to speeds in excess of 300 km/h in less time than it takes to read this sentence. Pointing to the end of the crankshaft on a partially disassembled engine Mäkinen smiles mischievously and says, “This is where we put the Renishaw magnetic actuator. We’ve located it directly on the end of the shaft. The encoder body is on the other side, in the clutch housing. I honestly think that only a space rocket would be a tougher environment for the encoder.”

The RM22 is designed and manufactured by Renishaw’s Slovenia-based partner company, RLS d.o.o. for high reliability in demanding applications and extreme environments. Non-contact designs—basically a magnet and a sensing chip—the robust, solid-state encoders feature a small actuator magnet that fits to the end of a shaft, while the Hall sensor ASIC chip is epoxy-embedded and encapsulated inside a metal housing/mount. Fully sealed to IP68, the encapsulated design provides superior resistance to shock, vibration, and acceleration/deceleration forces.

The encoder’s magnetic actuator mounts on the end of the engine crankshaft, slipping into the encoder body when the pit crew assembles the clutch housing. (Photo courtesy of Renishaw).

While maximum operating temperature is specified as 125 °C, the RM22 encoder still survives the very high temperature inside the clutch. The secret, says Mäkinen, is placing the encoder in the heavy, machined aluminum clutch housing and removing it fast. “With a diameter of only 22 mm, it was possible to locate the encoder in the clutch housing, itself, without compromising the strength of the piece. Clutches can explode,” he says, as an aside. “I’ve seen this happen a few times, so the clutch housing plays an important role in rider safety. If the encoder were bigger, we’d have to re-engineer the solution,” he notes.

“The heavy aluminum body shields the encoder from the heat during the race,” says Mäkinen. “After each race, our mechanics cool the clutch and quickly remove the housing and the encoder. Before it ever gets a chance to heat up beyond specs, we’ve already got it back to the pit garage and removed it. Given the conditions, the reliability of the Renishaw encoders is amazing,” he continues. “Next season, [they] will also be used to control the ignition advance and, in a couple of years, we are going to use the encoders to adjust cams. Without them, we could do nothing more than estimate vital engine and track information,” he says. “But guessing isn’t good enough if you want to break records.”

— C.G. Masi , senior editor, Control Engineering Daily News, (Register here and scroll down to select your choice of eNewsletters free.)